NONLINEAR SPIN DYNAMICS AND ULTRA-FAST PRECESSIONAL SWITCHING

Abstract

This thesis is intended to provide a theoretical analysis of magnetization dynamics in nanometer scale structures over picosecond time scales. This research has been motivated by promising technological applications in the area of magnetic data storage, as well as by pure scientific quest for ultra-fast spin dynamics in nanostructures.

The present paradigm of magnetic data storage is approaching its fundamental limits for areal storage density, as well as for speed in data processing. As a result, there is an urgent need for reliable alternatives to current magnetic recording media, which are based on longitudinal thin film, and to the conventional mechanism of magnetization reversal, based on damping switching. In this dissertation, faster modes of magnetization reversals, using precessional magnetization motion, are analyzed in traditional longitudinal media and in its promising alternatives: perpendicular and patterned media. This analysis uses multi-spin description of magnetic nanoparticles and continuum micromagnetics for thin film media. The spins dynamics in both discrete and continuum versions is modeled by Landau Lifshitz type equations. These models are introduced in Chapter 2, subsequent to an overview of magnetic recording media offered in Chapter 1.

The analytical study of precessional switching in perpendicular thin film media is presented in Chapter 3. The features of precessional magnetization switching and conventional magnetization reversal are compared, and the design of magnetic field pulses that guarantee precessional switching is discussed. In Chapter 4, the study of precessional magnetization switching in longitudinal thin film media is undertaken. After a short summary of the research studies on this topic, the inverse problem approach to the analysis of precessional switching in these media is presented. This approach leads to explicit expressions for the magnetic field pulses that guarantee the precessional switching.

The study of surface anisotropy effects on magnetization reversals in nanoparticles is presented in Chapter 5. The expressions for critical magnetic fields that guarantee the quasi-static and precessional reversals are analytically derived for the case of very strong exchange and weak surface anisotropy. These analytical results are also used to test the numerical approach, which is applied to the general case of the problem.